The present invention relates to weight level generation; and, in particular, to a method and device for generating weight levels that can be used for converting digital signals into analog signals.
In a conventional digital-to-analog converter (DAC), the quantization number for a digital signal must be consistent with the number of weight levels used when generating the analog signal, and the number of weight generators used for generating the analog signal must correspond to the number of weight levels. In particular, in the case of a configuration in which plural weight generators have equal weights in order to receive a thermometer code, it is desired that the number of said weight generators be consistent with the number of weight levels.
In the case of a typical DAC equipped with a conventional delta sigma (ΔΣ) modulator (that is, ΔΣ modulation DAC), the out-of-band noise is the remainder after noise shaping is performed by the ΔΣ modulator. The source of the out-of-band noise is the quantization noise. Since the amount of quantization noise is a function of the number of quantization levels, one method for reducing the amount of quantization noise is to increase the number of quantization levels. In this method, in order to increase the number of quantization levels, the number of weight generators (known as “analog segments”) in the DAC must be increased. The increase in the number of analog segments will inevitably increase the area on the IC chip occupied by such analog segments. Consequently, the method of reducing the out-of-band noise by increasing the number of quantization levels is limited from the standpoint of realizing a DAC with limited IC chip area.
On the other hand, another method of reducing the quantization noise is to increase the operating speed of the ΔΣ modulator (that is, the over-sampling frequency is increased). In this method, however, the analog performance of the DAC deteriorates. In other words, as the over-sampling frequency is increased, the switching rate for the analog segments also increases. As a result, the distortion in the analog segment output increases, leading to deterioration of the distortion performance. Consequently, the method of reducing the out-of-band noise by increasing the operating speed of the ΔΣ modulator is also limited from the standpoint of realizing a certain distortion performance for the DAC.